Because of the demand for smaller yet more complex electronic devices, the parts used in their fabrication have become smaller and the spacing of the attachment leads on the parts has also become smaller. To achieve the quality levels and throughput required to be a successful competitor in the marketplace for such devices, robotic assembly techniques are essential. The small spacing between attachment leads encountered with the smaller, more complex parts necessitates the use of vision control for the robotic systems to achieve the higher accuracy of placement of the parts required to achieve good yields at the soldering stage of circuits boards. The use of fiducial marks specific and adjacent to the locations of critical parts on the circuit boards is a method commonly used in conjunction with vision systems to achieve the required highly accurate horizontal placement position.
One approach to creating and reading fiduciary marks is to generate them with plating patterns on the top surface of a circuit board and use light reflecting into the vision system camera (mounted above the board) from a light source mounted above the circuit board. This approach has some drawbacks, the most significant of which is that the accurate reading of the mark generally requires a smooth, flat surface on the fiduciary mark formed from a material such as gold. When tin plating is used, the accuracy typically degrades because of the lower reflectivity and higher dispersion of the reflected light due to the unevenness of the surface.
An alternative approach is to use a light source located beneath the board, relying on the translucent nature of printed circuit board material, and then to design the vision system to analyze the shadow of the fiduciary mark. Because the edges of the shadow are quite sharp this can produce even better accuracy than that obtained with the top lit approach, when the light is directed perpendicularly to the plane of the board.
A problem in using the bottom lit approach occurs in assembly lines that use conveyer systems which employ belts to convey the circuit boards from station to station. In this system, when the board reaches an assembly station, it is typically lifted a small distance off the moving belts and held in a fixed position during an assembly stage. When a fixed bottom lighting system is used, fiduciary marks that are located over the belts will be illuminated only by light that is at an acute angle and diminished in brightness, both of which reduce the ability of the placement system to identify the fiduciary mark accurately. This leads to restrictive design rules concerning the locations on the circuit board where parts requiring high placement accuracy may be placed, generally leading to the undesirable result of larger boards and longer design times.
Thus, what is needed is a simple, reliable means of providing a light source in the small space that is created between the conveyer belt and the circuit board by lifting the circuit board in preparation for parts placement, and which will illuminate near the edges of the board as effectively as in the center of the board with a light source that is generally perpendicular to the board.